High-precision time synchronization is of applicable value in satellite navigation, aerospace industry, deep space exploration, geodesy, communication, and scientific research and measurements, etc. Traditionally, GPS common view or two-way satellite time transfer is employed in high-precision time synchronization. GPS common view requires relatively simple and low cost equipment, but is unable to achieve a precision of better than one nanosecond. Two-way satellite time transfer has a precision at the level of sub-nanosecond, but its stability, particularly in short-term, is prone to disturbance by the free space. So the precision of two-way satellite time transfer is difficult to be further improved, with further disadvantages in the security and high cost.
A fiber channel is highly stable, particularly within a short time scale, immune to outer influence, and has a low loss and high bandwidth. It first attracts researchers' attention in the 1980s. With rapid development in optical fiber communication and optical network technology and increased requirements for time synchronization precision, optical fiber based time synchronization achieves great progress and has been applied to certain extent.
High-precision fiber-optic time transfer has two major methods, the round-trip time transfer and the two-way time transfer. Round-trip time transfer is realized by means of stabilizing a link delay and calibrating the link propagation delay via returned signals from a far end. A 420 kilometer fiber-optic time and frequency transfer experiment based on the scheme has been demonstrated by AGH university of Polish with a time transfer stability of 1 ps/d. See Ł. Śliwczyński et al., “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420 km,” Metrologia, 50(2):133-145, 2013. Based on the similar scheme, the Laser Physics Laboratory in France reported a 540 kilometer time and optical carrier transfer experiment employing a satellite time and ranging equipment (SATRE) from the TimeTech in Germany, with a time transfer stability of 50 ps/s. See Lopez, Olivier et al., “Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network,” Applied Physics B110.1 (2013): 3-6.
Two-way fiber-optic time transfer based on WDM all-optical path is similar to two-way satellite time transfer. The clock difference between two sites is obtained by making use of path symmetry to eliminate link delay and its variation via simultaneously sending the local timing signals (1PPS) from the both ends to their counterparts. The CESNET in Czech reported a 744 kilometer two-way time transfer experiment over a single fiber based on the scheme, with a stability of 8.7 ps/500 s. See V. Smotlacha et al., “Time transfer using fiber links,” Proc. 24th European Frequency and Time Forum, Noordwijk, the Netherlands, 2010. The SP Technical Research Institute of Sweden carried out a 560 kilometer two-way time transfer over a domestic WDM optical network, and compared the synchronization result with the one of carrier phase, which is below 1 ns.
In order to suppress the influences from Raleigh backscattering and Fresnel reflection on the transmitted timing signals, both of the aforementioned schemes adopt bidirectional WDM transmission method (that is, different transmission optical wavelengths for two directions), and result in a bidirectional propagation delay asymmetry due to the fiber dispersion, which increases with the increase of distance and limits the precision of two-way time transfer over long distance. As regarding to the round-trip method, it is difficult to accurately calibrate the link delay for thousands of kilometers time transfer, due to the dispersion difference of fiber links in the practical networks. The PTB in Germany carried out a 73 kilometer two-way time transfer experiment by adopting the SATRE of the TimeTech. The method adopts the spread spectrum coding/decoding and realizes two-way time transfer over a single fiber with the same wavelength. However, high-precision spread spectrum coding/decoding is complex and has a high cost. See D. Piester et al., “Remote atomic clock synchronization via satellites and optical fibers,” Adv. Radio Sci., 9(1):1-7, 2011.